Charged particle flow control apparatus with apertured cathode



May 11, 1965 L. T. ZlTELLl 3,183,402

CHARGED PARTICLE FLOW CONTROL APPARATUS WITH-APERTURED CATHOJDE OriginalFiled Feb. 24, 1956 2 Sheets-Sheet l ill Lou/$7. Z/rezu uvvuvron avg fffii AITOP/VEV United States Patent F 3,183,402 CHARGED PARTICLE FLOWCONTROL APPA- RATUS WITH APERTURED CATHODE Louis T. Zitelli, Palo Alto,Calif., assignor to Varian Associates, Palo Alto, Calif., a corporationof California Original application Feb. 24, 1956, Ser. No. 568,422, newPatent No. 2,943,234, dated June 28, 1960. Divided and this applicationMay 5, 1960, Ser. No. 27,152

3 Claims. (Cl. 315-30) This invention relates in general to flow controlof charged particles and more specifically to novel high electricalcurrent density control electrodes and to novel circuitry networksuseful in conjunction with high power tube apparatus employing suchelectrodes.

The invention is extremely useful in the generation of high power, shortrise and fall time pulses as are utilized in radar, pulse communicationsystems and the like. In radar work, for example, the fall time shouldbe especially short so that in close range Work the returning echosignal will not be masked by the trailing edge of the out going pulse.

One system utilized in the generation of pulses, for example for highpower radar, employs a pulsed klystron amplifier operating into thetransmitting antenna. In this system the R.F. output of the klystron waspulsed by pulsing on and 0d the klystrons beam current. The beam currentwas pulsed by applying a positive going high voltage pulse to the anodewith respect to the cathode. The positive pulse in this system had to besubstantially equal to the beam voltage of the tube (often kv. or more).Short fall times, or" such high voltage pulses, have been extremelydifficult toachieve. These pulses have been plagued with long falltimes, overshoot, and ripple. In the past the pulse was generallyapplied to the anode rather than to a control grid because the controlgrids used in the prior art intercepted suflicient current to operate ata very high temperature such that there was thermal emission from thecontrol grid at a time when the tube was supposedly cut off therebyextending the fall time of the beam current pulses. Moreover, sporaticthermal emission from the hot grid during the beam current off periodproduced noise excitation in the output cavity resonator which wouldinterfere with the incoming echo signal. The present invention providesnovel improved means for the generation of short rise and fall time highpower pulses.

Accordingly, the principal object of the present invention is to providea novel high current density control apparatus whereby high power pulseshavingparticularly short rise and fall time characteristics may begenerated.

One feature of the present invention is a novel non intercepting currentcontrol electrode disposed in close proximity to the charged particleemitter and adjacent the flow of charged particles whereby the flow ofcharged particles from the emitter may be effectively controlled ormodulated with substantially no physical current interception by thecontrol electrode.

Another feature of the present invention is a novel current controlelectrode of annular configuration disposed in surrounding relationshipto a beam of charged particles and in close proximity to the particleemitte whereby high current density flow may be effectively controlledwithout physical current interception by the control electrode. I

Another feature of the present invention is a novel current controlmeans comprising a first control electrode disposed circumscribing thecurrent path and a second insulated control electrode within theconfines of the emitting surface of the charged particle emitterwherebyhigh 3,183,402 Patented May 11, 1965 ice view of a tube structureembodying the novel control elec-.

trode of the present invention,

FIG. 2 is an enlarged view of a portion of the structure of FIG. 1 takenalongline 2-2 in the direction of the arrows,

FIG. 3 is a fragmentary longitudinal cross sectional view partlyschematic showing a second novel current control electrode embodiment ofthe present invention,

FIG. 4 is a circuit diagram of .a novel pulse forming network,.

FIG. 5 is a graph of the potentials of certain electrodes as a functionof time of the circuit of FIG. 4,

FIG. 6 is a circuit diagram of another novel pulse form ing network, and

FIG. 7 is a graph of certain tube potentials as a function of time ofthe circuit of FIG. 6.

Similar characters of reference are used in all of the above figures toindicate corresponding parts.

The construction of the novel apparatus of the present invention willnow be described. Several embodiments are presented and each novelconstruction will be immediately followed by a description of itsoperation.

The present invention Will be described, to facilitate explanation, asit pertains to a pulse generating network employing a klystron amplifieras the output tube. It will be readily apparent to those skilled in theart that the scope of the present invention is not so limited and may beapplied to many systems employing other types of output tubes such astraveling wave amplifiers, etc., wherein it is desirable to preciselycontrol high current density flow.

Referring now to FIG. 1 there is shown a partial view of a high powerklystron amplifier which incorporates the novel control electrodestructure claimed in the copending parent application Serial No.568,422, filed February 24, 1956, and now US. Patent No. 2,943,234, ofthe present invention. A cathode assembly 1 is shown mounted upon and inaxial alignment with a multi-resonator R.F. section 2 which is disposedbetween the cathode assembly 1 and a collector assembly (not shown).

Included within the cathode assembly 1 is a cathode emitter 3. Anannular cathode focus electrode 4 encircles the outer periphery of thecathode emitter 3 in slightly spaced relation therefrom and, in thepresent instance, is electrically tied to the cathode emitter. Althoughin the instant case the focus electrode 4 is at the same potential asthe emitter 3, this is not a requirement and often will be found to beat aslightly different potential to give the desired focusing of thebeam.

A cylindrical control electrode support 5 is rearwardly disposedoutwardly and concentrically of the cathode emitter 3. A hollowcylindrical dielectric insulator 6 as of, for example, alumina ceramicis mounted on the forward outwardly flanged end of the control electrodesupport 5. The insulator 6 concentrically surrounds the cathode emitter3. An annular control electrode 7 is carried transversely of and uponthe forward end of the insulator 6. A control electrode lead 8 isconnected to the control electrode 7 and provides a means for applying avoltage to the control electrode which is independent of the voltageapplied to the cathode emitter 3.

The spacing between the mutually opposing portions of the focuselectrode 4 and the current control electrode 7 is made as small aspossible to give an eifective control over the current flow. In thecathode configuration shown in FIG. 1 this spacing is approximately0.015". The electrode spacing dimensions cited here are to be consideredonly exemplary and not in a limiting sense since the spacing that can betolerated using a certain electrode configuration will depend upon theoperating voltages of the opposing electrodes and the strength of theparticle accelerating field.

' The inside periphery of the apertured current control electrode 7 at Gand the forward end of the focus electrode at F have been rounded toprovide a relatively large radius of curvature attheir closest mutuallyopposing portions. Moreover the surface of these electrodes at therounded portions G Iand F. have been highly polished to prevent sharppoints-which would quite likely produce electric arcs between the focuselectrode 4'and the current control-electrode 7 when high voltagedifferences were encountered in use.

An outer cathode envelope 9surrounds the cathode emitter 3 and providesa gas-tight housing whereby the interior of the cathode assembly 1 maybe evacuated. A

transverse central-1y apertured anode pole piece 11 carries 7 thecathode envelope 9. The cathode assembly is positioned in axialalignment with the central anode aperture. A .portion of the anode polepiece 11 is of magnetic material forming one pole of a permanent magnet,the yoke of which is not shown, which provides an axial focusingmagnetic fiield whereby the electrons are confined in a beam shape asthey proceed toward the collector end of the anode pole piece 11. Thisestablishes a certain voltage potential gradient between cathode emitter3. and anode pole piece 11 which is sufiicient to accelerate electronsemitted from the cathode 3 through the centrally apertured anode polepiece 11. When a sufiiciently more negative potential than the cathodepotential is applied to. the'current control electrode 7, a negativepotential barrier is established between cathode 3 and anode .11 whichwill prevent the emitted electrons from being acted upon by the positiveaccelerating potential applied to the anode 11, thereby preventing theflow of beam current.

The degree to which the potential applied to the control electrode 7must be more negative than the potential applied to the cathode 3dependsupon the strength of the accelerating field and'the configuration anddisposition of the control electrode 7. The control electrodeconfiguration and disposition shown in FIG. 1, to effectively inhibitbeam current, requires a control electrode voltage more negativethan'the cathode'potential of approxi-.

mately 60% of the potential difference between cathode '3 and anode 11.For example; if the anode-to-cathode voltage is 10 kv. the potential ofthe control electrode 7 must be 6 kv. more negative than the cathodepotential.

The focus electrode 4 acts in the conventional manner elocitymoduleitethe beam. The beam proceeds through successive intermediate cavityresonators (not shown) which further velocity modulate the beam. Thencethe electrons enter an output cavity resonator wherein they impartelectromagnetic energy to the cavity resonator.

The electromagnetic energy is then coupled out of the output resonator(not shown) and propagated to the load as, for example, a transmittingantenna (not shown).

Referring now to FIG. 3 there is depicted an embodiment of the presentinvention. Herein an apertured charged particle emitter 14 having aspherically concave emitting surface is provided and will deliver aslightly hollow beam which sometimes is preferred for certain types oftubes. A hollow cylindrical current control electrode 15 is positionedin concentric surrounding and insulated relationship to the aperturedcathode emitter 14 and has a free end portion overhanging the emittingsurface of the cathode emitter 14.' A central current control electrode16 protrudes through the central aperture in the emitter 14 and isinsulated electrically from said emitter 14. Although a centralelectrode is depicted this electrode need not be centrally disposed, forexample, it could be a second hollow cylindrical electrode protrudingthrough the emitter in a concentric fashion.

The cylindrical control electrode 15 and the central electrode 16 areshown electrically tied together and thus operate ja'tsubstantially thesame electrical potential.

However, for some applications it may be desirable to have the twoelectrodes operating at different potentials. A cathode lead 17 providesan independent potential to the emitter 14. An anodelead supplies a morepositive potential to acentrally apertured accelerating-anode 18 than isapplied to the cathode emitter 14. The anode 18 is characterized byhaving but a single aperture in axial alignment with the emitter 14. Acurrent control electrode lead 8 supplies the operating potential to thecontrol electrodes 15 and 16.

Surface discontinuities of 'the control electrodes and other tubeelectrodes operating at high voltages are made to have relatively largeradii of curvatures whereby points of extremely high electric fields areminimized. Moreover the electrodes are polished to further prevent arcsbetween electrodes operating at different potentials. A'

hollow cylindrical dielectric insulator 19 as of, for example, aluminaceramic is disposed between the central control electrode 16 and theemitter 14.

InoP ation, the novel two-member current control electrode 15, 16operates similarlyto the previously men- Itioned annular current controlelectrode ,7 and focusing. electrode 4 combination. When the tube isdrawing beam on the cathode 14, a negative potential barrier is estab-'lished between the cathode 14 and the anode 18 whereby beam current iseffectively cut off. The instant two-memher current control electrodeconfiguration. requires approximately only half of thepotentialdifference between cathode and control electrode as, required using thesingle control electrode 7. i

The previously described electrode configurationsmay be utilized toadvantage in controlling many high' cure rent density flow'devices. Thenetworks for establishing the desired operating potentials onv thecertain electrodes may be of varied form depending upon the desiredobjective of the apparatus, for example, modulating, pulse fall timepulses and forming the subject matter of a copending divisionalapplication Serial No. 27,151, filed May 5, 1960, out of the same parentapplication: Serial No. 568,422, which parent'has now issued as U.S.Patent No.2,943,234. An output tube 21 as, :for example, a multicavityklystron amplifier, as described supra, is

connected in a first series circuit to a second modulating tube 22 as,for example, a power triode having a plate 23, a cathode 24, and acontrol grid 25. The plate 23,

is connected to a cathode 26 of the power tube 21. A' potential dividerbranch 27 consisting of a first resistor R and a second resistor R isparallel connected with the first series circuit or branch. A tap T ofthe potential divider, branch is connected to the cathode 26 of thepower tube 21. One end of the potential divider branch 27 is connectedto an accelerating electrode 28 of the power tube 21. The other end ofthe potential divider branch is connected to the cathode 24 of themodulator tube.

The accelerating electrode 28 of the power tube 21 is connected toground. The cathode 24 of the modulator tube is connected to a certainpotential which is substantially more negative than ground, for example,10.5 kv. A current control electrode 22 of the power tube 21 isconnected to a potential slightly more positive than the cathode 24 ofthe modulator tube 22, as of, for example, at 10.0 kv. The control grid25 of the modulator tube 22 is biased at cutoff. The current flowthrough the potential divider branch 27 establishes a potential on thecathode 26 of the power tube 21 substantially more positive than thepotential of its current control electrode 29 as of, for example, -6.5kv.

In operation, before any initiating signals are intro duced, both thepower tube 21 and modulator tube 22 are biased at cutoff. The onlycurrent flowing is through the potential divider branch 27 whichestablishes the potential of the cathode 26 of the power tube 21.

When a positive going initiating pulse is received at the grid 25 of themodulator tube 22 the beam of the modulator tube 22 is turned on and themodulator tubes effective resistance diminishes to a small amountthereby establishing the cathode 26 of the power tube 21 at a potentialslightly higher than the potential of the cathode 24 of the modulatortube 22. For example, when the modulator tube is turned on the cathodeof the power tube 26 is dropped to approximately l0.0 kv. The po tentialof the current control electrode 29 of the power tube 21 remains fixedat -l0.0 kv. Thus at this time 10.() kv. exists between the power tubesaccelerating electrode 28 and its cathode 26 and no potential barrierexists due to the current control electrode 29. The beam of the powertube 21 is turned on. An RF. signal applied to the input cavity of thepower tube 21 is then amplified and propagated to the load.

When the end of the positive going initiating pulse arrives at the grid25 the modulator tube 22 is cut off. This initiates the return of thecathode 26 of the output tube 21 to the voltage divider bias condition,for example, in the instant case, 6.5 kv. When the cathode ot the powertube 21 reaches a sufficiently more positive potential than its currentcontrol electrode 29 which remains at l0.0 kv. the power tube is cut ofiand the RF. signal can no longer be amplified and thus the RF. outputpulse is terminated.

At this point it is necessary to examine more carefully thecharacteristics of the pulse generating circuit. Upon a closer analysisof the modulator tube 22 it will be found that there are certainelectrode capacitances and stray wiring capacitances associated with themodulator tube 22 and its attendant wiring. This capacitance can belumped into an equivalent capacitance represented by capacitor Cshunting the modulator tube 22. The electrical effect of this shuntingcapacitance is to draw current to charge the capacitor C when a voltageis suddenly applied across its terminals. The charging current in thepresent network is primarily drawn through the output tube 21 as beamcurrent. Thus the beam current of the output tube 21 falls offexponentially rather than cutting off instantaneously. Another way tolook at this is to say the voltage goes more positive across thecapacitor as shown by the following relationship:

where V is the instantaneous voltage across the capacitor C V is theapplied potential difference in the network, 1' is the time in seconds,R is the resistance through which the charge must flow to charge thecapacitor C0, and C is the capacitance of the capacitor C This means thepotential of the power tubes cathode 26 will raise in 6 substantially anexponential manner thus causing the tall time of the beam current pulseto be lengthened a small amount over zero fall time.

The fall time of the beam current pulse, as can be seen from the aboverelationship, varies directly with the resistance through which thecapacitor C charging current must flow. In the circuit of FIG. 4 thevalue of R is equal to the resistance of the conducting power tube (r inparallel with the first potential divider resistors R and R Thus thetotal resistance 1 l 1 R 1 Tp R 2 out assuming R and R are individuallymuch, much greater than r then R r See FIG. 5 for a graphof certainaforementioned electrode potentials as a function of time.

A second novel pulse generating circuit is shown in FIG. 6 wherein thefall time characteristics of the pulse are considerably improved. Thiscircuit also forms the subject matter of the aforementioned copendingdivisional application Serial No. 27,151. As in the previous network ofFIG. 4 an output tube 21, as, for example, a high power klystronamplifier is placed in series with a pulse modulating tube 22. A smallreference resistor 31 as of, for example, 70 ohms is interposed in theseries circuit between the output tube 21 and the modulating tube 22. Arestorer tube 32 having a control grid 33, plate 34, and cathode 35 as,for example, a 465A (a tetrode) is provided having the referenceresistor 31 series connected in its cathode-to-grid circuit. In this waywhen full beam current flows through the series branch it will develop apotential drop across the reference resistor 31 which will negativelybias the third tube 32 below cutoff.

The plate 34 of the restorer tube 32 is connected to a constantpotential source 36 intermediate of the modulator cathodes low potentialand ground as, for example, 6.5 kv. As in the previous circuit thecurrent control electrode 29 of the output tube 21 is set at a certainpotential more positive than the cathode 24 of the modulator tube 22 as,for example, l0.0 kv. The accelerating electrode 28 of the output tube21 may be set at ground potential. The modulator tube 22 is biased atcutofi.

A positive going pulse applied to the grid of the modulator tube 22turns its beam current on and immediately drops the cathode 26 of theoutput tube 21 to appoximately l0.() kv. This immediately turns on thebeam current in the output tube 21. An RF. signal applied to the inputcavity is amplified and propagated to the antenna.

When the end of the positive going initiating pulse arrives at the grid25 of the modulator tube 22, the modulator tube 22 again returns to thecutofi state and current through the series branch terminates. When theseries current stops there is no voltage drop across the referenceresistor 31 and thus no negative bias on the grid of the restorer tube32. Hence the restorer tube 32 draws space current and tends toimmediately raise the potential of the cathode 26 of the output tube 21to the potential of the plate 34 of the third tube 32 (6.5 kv.).

However, here again the capacitance C shunting the modulator tube 22would tend to cause the cathode voltage of the output tube 21 to raisein an exponential manner as shown by the previously describedrelationship In this network R in the above relationship is reduced overthe first novel circuit. Thus the fall time is less than that for thefirst circuit because the current to charge the capacitor C may comethrough the low resistance 4,, of the restorer tube 32 as well as fromthe output tubes beam current. In other words, the total and if r =rthen -R /2r or half the resistance of the first described network whichshould give one-half the fall time of the first circuit.

Decreasing the fall time of the beam current pulse likewise cuts the RF.pulse off at a faster rate resulting in the desired short fall timecharacteristics of the RF. output'pulse.

This application is a divisional of my copending application, Serial No.568,422, filed February 24, 1956, now .Patent No, 2,943,234, for ChargedParticle Flow Control Apparatus.

Since manychanges' could be made in the above construction and manyapparently widely difterent embodimentsof this invention could be madewithout departing fromthe scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not ina' limiting sense.

a What is Claimed is:

w 1. A gated electron gun assembly for high power high frequencyvelocity modulation electron tube apparatus including; an aperturedelectron emitter having a concave emitting surface closely approximatinga spherical segment for emitting a'high current beamyan acceleratingelectrode facing said concave surface of said emitter and having but asingle aperture in axial alignment with said emitter through which isdrawn a substantially solid converging beam of electrons from saidemitter in a substantially non-intercepting manner with the margin ofsaid aperture in said accelerating electrode, a first apertured, currentcontrol electrode electrically insulated from said emitterhaving a\portion disposed between said accelerating'electrode and said emitterand disposed in close spatial proximity to said electron emitter and inencircling spatial relationship to the emitting surface of said electron8 emitter, a second electron control electrode mounted inaxialfalignment with the aperture in said electron-emitterandelectrically insulated from said emitter, and means for applying only anegative variable potential to said controlielectrodes with respect tosaid electron emitter whereby .the flow of electrons from said electronemitter may beeifectively gated without said current control electrodesphysically intercepting the electron beam to produce undesired thermalemission therefrom, in use.

2 Anapparatus as claimed in claim 1 wherein said first current controlelectrode is a cylindrical member encircling the outside edge of saidelectron emitter and has a portion overhanging the emitting surface ofsaid electron emitter.

3. An apparatus as claimed in claim 2 wherein the aperture in saidelectron emitter is cenerally disposed,

and said second current control electrode member is:

mounted in the central aperture and projects from the concave emittingsurface of said electron emitter and is adapted to hold a potentialindependent of said electron emitter, whereby through the combinedaction of said cylindrical currentv control electrode and said secondcentrally disposed current control electrode the flow of particles fromsaid emitter may be effectively gated.

Reterences Qited by the Examiner V UNITED STATES PATENTS ROBERT SEGAL,Acting Primary Examiner. ARTHUR GAUSS, GEORGE N. WESTBY, Examiners.

Jepsen'"; 3l5-5.39 X

1. A GATED ELECTRON GUN ASSEMBLY FOR HIGH POWER HIGH FREQUENCY VELOCITYMODULATION ELECRON TUBE APPARATUS INCLUDING, AN APERTURED ELECTRONEMITTER HAVING A CONCAVE EMITTING SURFACE CLOSELY APPROXIMATELY ASPHERICAL SEGMENT FOR EMITTING A HIGH CURRENT BEAM, AN ACCELERATINGELECTRODE FACING SAID CONCAVE SURFACE OF SAID EMITTER AND HAVING BUT ASINGLE APERTURE IN AXIAL ALIGNMENT WITH SAID EMITTER THROUGH WHICH ISDRAWN A SUBSTANTIALLY SOLID CONVERGING BEAM OF ELECTRONS FROM SAIDEMITTER IN A SUBSTANTIALLY NON-INTERCEPTING MANNER WITH THE MARGIN OFSAID APERTURE IN SAID ACCELERATING ELECTRODE, A FIRST APERTURED, CURRENTCONTROL ELECTRODE ELECTRICALLY INSULATED FROM SAID EMITTER HAVING APORTION DISPOSED BETWEEN SAID ACCELERATING ELECTRODE AND SAID EMITTERAND DISPOSED IN CLOSE SPATIAL PROXIMITY TO SAID ELECTRON EMITTER AND INENCIRCLING SPATIAL RELATIONSHIP TO THE EMITTING SURFACE OF SAID ELECTRONEMITTER, A SECOND ELECTRON CONTROL ELECTRODE MOUNTED IN AXIAL ALIGNMENTWITH THE APERTURE IN SAID ELECTRON EMITTER AND ELECTRICALLY INSULATEDFROM SAID EMITTER, AND MEANS FOR APPLYING ONLY A NEGATIVE VARIABLEPOTENTIAL TO SAID CONTROL ELECTRODES WITH RESPECT TO SAID ELECTRONEMITTER WHEREBY THE FLOW OF ELECTRONS FROM SAID ELECTRON EMITTER MAY BEEFFECTIVELY GATED WITHOUT SAID CURRENT CONTROL ELECTRODES PHYSICALLYINTERCEPTING THE ELECTRON BEAM TO PRODUCE UNDESIRED THERMAL EMISSIONTHEREFROM, IN USE.